Stationary and transport applications of air conditioning and refrigeration systems control the temperature of a conditioned space to a predetermined temperature range adjacent to a selected set point temperature, with transport applications including those associated with a vehicle, such as a straight truck, a tractor-trailer combination, a refrigerated container, and the like. Such systems conventionally utilize a chlorofluorocarbon (CFC) refrigerant in a mechanical refrigeration cycle. The mechanical refrigeration cycle requires a refrigerant compressor driven by a prime mover, which often includes a dedicated internal combustion engine, such as a diesel engine. Because of the suspected depleting effect of CFC's on stratospheric ozone (O.sub.3), practical alternatives to the use of CFC's are being sought.
The use of a cryogen, i.e., a gas which has been compressed to a very cold liquid state, such as carbon dioxide (CO.sub.2) and nitrogen (N.sub.2), in air conditioning and refrigeration systems is particularly attractive because, in addition to eliminating the need for a CFC, it also eliminates the need for a compressor and associated prime mover. Air conditioning and refrigeration systems of which we are aware which utilize a cryogen, implement a cooling cycle by circulating the cryogen through a fluid path which includes a heat exchanger which is in heat exchange relation with air from a conditioned space. When a heating cycle is required to hold the temperature of the conditioned space within a predetermined narrow temperature band adjacent to a selected set point temperature, or a defrost cycle is required, the cryogen is heated by a burner and associated fuel, and the heated cryogen is circulated through the fluid path and heat exchanger. Thus, cryogen is expended to the atmosphere during a cooling cycle, and cryogen plus a fuel, such as propane, diesel fuel, liquid natural gas, and the like, are expended to the atmosphere to implement heating and defrost cycles.
It would be desirable, and it is an object of the present invention, to provide new and improved cryogenic based air conditioning and refrigeration systems, which more effectively and efficiently utilize the cryogen, for lower cost operation, as well as for an extended operating time for a given vessel of cryogen.